22 THE CHEMISTRY OF METAMOEPHIC ROCKS. [III. 



orthoclase, but to the well-known power of argillaceous sedi- 

 ments to abstract from water the potash- salts which it already 

 holds in solution. Thus when a solution of silicate, carbonate, 

 sulphate or chloride of potassium is filtered through common 

 earth, the potash is taken up, and replaced by lime, magnesia, 

 or soda, by a double decomposition between the soluble potash- 

 salt and the insoluble silicates or carbonates of the latter bases. 

 Soils, in like manner, remove from infiltrating waters, ammonia, 

 and phosphoric and silicic acids, the bases which were in combi- 

 nation with these being converted into carbonates. The drain- 

 age-water of soils, like that of most mineral springs, contains 

 only carbonates, chlorides and sulphates of lime, magnesia and 

 soda ; the ammonia, potash, phosphoric and silicic acids being 

 retained by the soil. 



The elements which the earth retains or extracts from waters 

 are precisely those which are removed from it by growing 

 plants. These, by their decomposition under ordinary condi- 

 tions, yield their mineral matters again to the soil ; but when 

 decay takes place in water, these elements become dissolved, 

 and hence the waters from peat-bogs and marshes contain 

 large amounts of potash and silica in solution, which are carried 

 to the sea, there to be separated, the silica by protophytes, 

 and the potash by algae, which latter, decaying on the shore, 

 or in the ooze at the bottom, restore the alkali to the earth. 

 The conditions under which the vegetation of the coal-formation 

 grew and was preserved being similar to those of peat, the 

 soils became exhausted of potash, and are seen in the fire-clays 

 of the carboniferous period. 



Another effect of vegetation on sediments is due to the re- 

 ducing or deoxidizing agency of the organic matters from its 

 decay. These, as is well known, reduce the peroxide of iron 

 to a soluble protoxide, and remove it from the soil, to be 

 afterwards deposited in the forms of iron-ochre and iron-ores, 

 which by subsequent alteration become hard, crystalline and 

 insoluble. Thus, through the agency of vegetation, is the iron- 

 oxide of the sediments withdrawn from the terrestrial circula- 

 tion ; and it is evident that the proportion of this element 



